DETERMINING TEMPERATURE-INVARIANT ENTHALPY CHANGE AND OTHER THERMODYNAMIC FUNCTIONS ON TRANSFORMATION OF PROTEINS AND OTHER BIOPOLYMERS

To gain insight into the thermodynamic changes during the chemical rea ctions or phase transformation of bioploymers, particularly the unfold ing of proteins (or their association) which occurs with exceptionally large enthalpy and entropy changes, a procedure suggested by Benzinge r (Benzinger, T, H. Nature (London) 1971, 229, 100), and variously use d by Chun (J. Phys. Chem. 1996, 100, 7283. Ibid. 1994, 98, 6851), has been revised in the light of the observations that proteins vitrify gr adually on cooling and behave like a glass. Thus a protein retains a f inite entropy at 0 K. This makes the method suggested by Benzinger unt enable for determining the temperature-independent energy change due t o the breaking and reforming of chemical bonds in proteins and other b iopolymers during their chemical and physical transformations. The val ue of the temperature-invariant enthalpy deduced already by this metho d are underestimated by a term equal to T Delta S-0, i.e., the tempera ture at which the Gibbs free energy change is zero (or the equilibrium constant is unity), multiplied by the difference between the residual entropy of the two states (or products and reactants). A resolution o f the issue of the unusually large enthalpy change on chemical and oth er transformations in proteins has been given in terms of configuratio nal contributions to the enthalpy and entropy of proteins. These contr ibutions arise from an exceptionally large number of the degrees of mo lecular freedom in the biopolymer and the diffusion of water and that of the substances added to facilitate protein association or unfolding . The procedure has been illustrated from calculations of several ther modynamic state functions for an orientationally disordered crystal wh ich shares the characteristic behavior of crystalline proteins in as m uch as the occurrence of molecular configurational freedom and the kin etic freezing of the degree of this configurational freedom, or glassl ike transition, is concerned. The calculations and the underlying conc epts transcend the details of the molecular structure of a material.